As cellular radiotelephony has become more popular, cellular service providers have felt increasingly pressured to use the cellular RF spectrum as efficiently as possible. Greater efficiency allows a service provider to carry more calls using a given amount of RF spectrum. However, the cellular industry generally accepts that when a channel is assigned to a first call, that channel and other channels should not be assigned to other calls in a manner which will cause interference with the first call. Accordingly, interference potential causes service providers to impose constraints on channel usage which limit the efficient usage of spectrum.
The cellular industry has relied upon interference measurement parameters, such as carrier-to-interference (C/I) ratio, to indicate the interference potential of particular channel assignment patterns. Typically, when the C/I ratio for a given channel used in a given cell exceeds a predetermined minimum, typically set at about 18 dB, voice quality is deemed acceptable. Hence, channels are assigned to cells in a manner which ensures that each channel in each cell will meet or exceed a minimum predetermined C/I ratio, assuming all channels are concurrently in use. Other constraints, such as a minimum reuse distance, adjacent channel constraints and co-site constraints, that are consistent with exceeding the minimum predetermined C/I ratio may be also be imposed. As long as established constraints are met, each channel is assigned to as many cells as possible within a cellular system, and voice quality remains acceptable even if all channels are concurrently used wherever they may be assigned throughout the cellular system.
Spectrum usage efficiency improvements may be achieved by devising channel assignment schemes that allow more channels to be used in each cell. A variety of conventional channel assignment schemes have been devised with the aim of efficiently using the RF spectrum. A fixed channel assignment (FCA) scheme is commonly used. In fixed channel assignment, constraints are established consistent with exceeding some minimum predetermined C/I ratio. Channels are assigned to as many cells as possible within a system in a manner which makes all channel assignments consistent with the constraints. The channel-to-cell assignments are maintained for a relatively long period of time (i.e. typically much longer than the duration of a call). As call requests arise in the cells where channels are assigned, the channels are assigned to the calls. When calls terminate or are handed off to other cells, the channels used to service the calls remain assigned to their cells.
A dynamic channel assignment (DCA) scheme may take a pure or hybrid form. With pure DCA, all channels are held in a central pool until a call request arises in a particular cell. At that instant, a channel is selected from the central pool and simultaneously assigned to the cell and the call. The selected channel meets predetermined assignment constraints. For example, a channel may be selected from the central pool of channels only if it exceeds the minimum predetermined C/I ratio. Hybrid DCA is a hybrid of FCA and pure DCA. Some channels are assigned to cells indefinitely using FCA and others are assigned as the need arises using DCA. Both pure and hybrid forms of DCA achieve spectrum efficiency over FCA. Pure DCA achieves the most improvement in spectrum efficiency when intra-cell handoffs are allowed.
A channel borrowing assignment scheme indefinitely assigns channels to cells independent of call activity, in a manner similar to FCA. When all assigned channels in a cell are used, the next call may be assigned to a channel borrowed from a nearby cell. However, before a channel may be borrowed from a nearby cell, that channel must not currently be in use or otherwise locked from being borrowed. If a channel is borrowed, that channel is then locked to prevent its use in the borrowed-from cell to ensure that the minimum predetermined C/I ratio will be maintained. When the borrowed channel is released in the borrowing cell, the channel is unlocked in the borrowed-from cell.
An underlay/overlay channel assignment scheme also indefinitely assigns channels to cells independently of call activity, in a manner similar to FCA. Large overlaid cells share radio coverage area with small underlaid cells but have different channels assigned thereto to prevent interference. Due to the small size of the underlaid cells, the channels assigned thereto may be reused at smaller distances than the channels assigned to the larger overlaid cells while maintaining the same minimum predetermined C/I ratio. The underlay/overlay scheme combines a high-power and less spectrally efficient cellular system with a low power and more spectrally efficient cellular system. Calls are handed off between the two systems.
In a related technique, a cell site may measure interfering signals present on a channel immediately prior to assigning a call to the channel. If, in spite of any channel to cell assignment scheme, interference appears to be below the threshold acceptability level (i.e. a C/I ratio of &lt;18 dB), that channel is not assigned.
Each of the above-discussed conventional channel assignment schemes identifies a channel which may be assigned to a call. That channel may come from a central pool, be borrowed from a nearby cell or be assigned to an underlay/overlay cell. From the perspective of the service provider, each channel provides acceptable voice quality and is therefore indistinguishable on the basis of voice quality from other channels which might have been assigned. Measurements may be taken to confirm that voice quality will be acceptable. Any channel assigned to a call provides acceptable voice quality up to the point where all channels, including all borrowed channels and underlaid or overlaid channels which share radio coverage area, are in use. At that point, the next call is blocked because communication services cannot be provided to the next call. If interference measurements indicate too much interference on a channel, that channel will not be assigned to a call even if the result is call blockage.
The conventional channel assignment schemes are undesirable because they achieve only a limited amount of spectral usage efficiency over FCA. In addition, such schemes are undesirable because they fail to gracefully degrade communication service as communication traffic increases. Rather, they all achieve roughly equivalent service quality up to the point of call blockage, at which point they fail to provide communication services for blocked calls.